1. Field of the Invention
The invention relates to oil and gas exploration/production. More particularly, the invention relates to a method and apparatus for improved communication techniques between an uphole apparatus and a downhole tool.
2. State of the Art
Logging-while-drilling (LWD) or measurement-while-drilling (MWD) involves the transmission to the earth's surface of downhole measurements taken during drilling. The measurements are generally taken by instruments mounted within drill collars above the drill bit in order to obtain information such as the position of the bit, oil/gas composition/quality, pressure, temperature and other geophysical and geological conditions. Indications of the measurements must then be transmitted uphole to the earth's surface. It has been one longstanding objective to develop data transmission systems which do not require the utilization of electrical conductors. The utilization of electrical conductors has several serious disadvantages including: (1) since most wellbores include regions which are exposed to corrosive fluids and high temperatures, a long service life cannot be expected from a data transmission system which utilizes electrical conductors; (2) since most wellbores extend for substantial distances, data transmission systems which utilize electrical conductors are not generally considered to be cost effective, particularly when such systems are utilized only infrequently, or in a limited manner; (3) since all wellbores define fairly tight operating clearances, utilization of a wireline conductor to transmit data may reduce or diminish the operating clearance through which other wellbore operations are performed; and (4) since wellbores typically utilize a plurality of threaded tubular members to make up tubular strings, utilization of an electrical conductor to transmit data within the wellbore complicates the make-up and break-up of the tubular string during conventional operations.
Accordingly, the oil and gas industry has moved away from the utilization of electrical conductor data transmission systems (frequently referred to as "hardwire" or "wireline" systems), and toward the utilization of wireless systems to transmit data within the wellbore. The most common scheme for transmitting measurement information utilizes the drilling fluid within the borehole as a transmission medium for acoustic waves modulated to represent the measurement information. Typically, drilling fluid or "mud" is circulated downward through the drill string and drill bit and upward through the annulus defined by the portion of the borehole surrounding the drill string. The drilling fluid not only removes drill cuttings and maintains a desired hydrostatic pressure in the borehole, but cools the drill bit. In a species of the technique referred to above, a downhole acoustic transmitter known as a rotary valve or "mud siren", repeatedly interrupts the flow of the drilling fluid, and this causes a varying pressure wave to be generated in the drilling fluid at a frequency that is proportional to the rate of interruption. Logging data is transmitted by modulating the acoustic carrier as a function of the downhole measured data.
One type of MWD technique called Vertical Seismic Profiling (VSP) involves the use of a seismic source and sensors, together with a memory and calculation device for storing and processing the received seismic signals. U.S. Pat. No. 5,585,556 describes a method and apparatus for performing VSP-measurements where the seismic source is placed at or in the vicinity of the surface of the earth (or water). Signals generated by the source are detected by hydrophones or geophones located in the vicinity of the source at the surface and in the drill string. The geophones or the hydrophones in the drill string transmit the detected signals to a memory and calculation unit in the drill string that processes and transmits the signals completely or partly to a central data processing unit on the rig. The hydrophones or the geophones at the surface simultaneously transmit the detected signals to the central data processing unit at the surface, while chronometers showing identical times, connected to the source and to the memory and calculation device in the drill string make possible a precise calculation of the travel time of the seismic signal between the source and the geophones or the hydrophones in the drill string.
According to the preferred embodiment of the '556 patent, circulation of the drilling fluid is interrupted as the sensors in the drill string are activated for the registration of sound signals discharged from the seismic source. In the following 60-120 seconds the memory and calculation unit will acquire all signals from the sensors in the drill string. The signals contain both the transmitted and the reflected waveforms. The sources must be discharged within a fixed interval of time. After this time interval, the content of the memory unit is copied to the calculation unit and is processed to determine the number of shots, the mean arrival time, and the mean amplitude of the first arrived signals. This information may be returned to the surface using MWD telemetry while the drilling fluid is put into circulation again. Alternatively, the information may be stored in memory and retrieved when the tool is tripped out of the borehole.
A disadvantage of the method disclosed by the '556 patent is that there is no disclosed method of communicating with the downhole tool to indicate to the downhole tool that VSP shots are commencing. Since the downhole data acquisition is non-discriminatory when circulation is stopped, i.e., most of the data recorded does not include useful signal information; the downhole tool continuously receives and stores acoustic waveforms without any disclosed discrimination. Since MWD telemetry has a very small bandwidth, e.g., one bit per second, transmission of useless information can delay the transmission of valuable data from other LWD tools. Ultimately, time is wasted and since the cost of rig time, particularly offshore rig time, is extremely expensive, time is of the essence when acquiring downhole measurement information. Moreover, while it is possible to store all of the acquired data in downhole memory, most of the memory will be wasted and the tool will need to be withdrawn more frequently than desired.
U.S. Pat. No. 5,579,283 describes a method and apparatus for communicating coded messages in a well bore. The method uses transmitting and receiving apparatus that are in contact with the well bore fluid to send coded pressure pulses to downhole tools. A disadvantage of the method described by the '283 patent is that the communication technique alters the operating state of the downhole tool.
Thus, there remains a need for a technique to communicate with a downhole tool in an efficient manner that discriminates between signal data, representative of a true signal originating from a selected signal source, and useless data.